Method of removing a silver and phenolformaldehyde resin paint from an article



Sept. 1, 1964 J, R. GOODIN 3,147,148

METHOD OF REMOVING A SILVER AND PHENOL-FORMALDEHYDE RESIN PAINT FROM ANARTICLE Filed Aug. 28, 1962 N \TQIC ACID 2 WASH 3 SOD\UM HYPOCHLOEWE 4WASH 5 DEY \NVENTOE OODIN ATTORNEY United States Patent Ofitice3,147,148 Patented Sept. 1, 1964 3,147,148 METHGD F REMOVING A SILVERAND PHENOL- FORMALDEHYDE RESEN PAINT FROM AN ARTICLE Jim R. Goodin,Gklahoma City, Okla, assignor to Western Electric Company, Incorporated,New York, N.Y., a corporation of New York Filed Aug. 28, 1962, Ser. No.21%,978 2 Claims. (Cl. 134-28) This invention relates to a method ofremoving a dried paint consisting of silver and a phenol-formaldehyderesin from an article, and more particularly to methods of removing suchpaint from deposited carbon resistors. A general object of the inventionis to provide a new and improved method of such character.

In the manufacture of deposited carbon resistors, a coating of carbonhaving a controlled thickness is deposited on the surface of acylindrical ceramic core by the decomposition of methane gas. To permitelectrical connection, a conductive paint is then applied to the ends ofthe resistors and baked on. This paint commonly includes a nonvolatilecontent consisting of about 80 to 92% by weight particulate silver withthe balance being a phenol-formaldehyde resin. In the practice of thismanufacturing process, the resistors produced sometimes fail to meetqualitative standards because either the silver and phenol-formaldehydecoating or the deposited carbon coating is imperfect. Since both theceramic cores and the carbon film deposited thereon are expensive it ishighly desirable and advantageous to remove the baked silver andphenol-formaldehyde resin paint and reuse the cores when either theconductive coating or the carbon coating is faulty.

When deposited carbon resistors are manufactured in large quantities bytechniques of mass production, the number of units rejected for failureto meet qualitative standards may also be large. Therefore, in removingthe baked silver and phenol-formaldehyde resin paint from defectiveresistors to reclaim the cores, it is important to use a method which iseconomical as Well as elfective. The method should require a minimum oflabor and should use chemical ingredients which are economical andreadily obtainable.

Accordingly, another object of the invention is to provide a method ofremoving a dried paint consisting of silver and a phenol-formaldehyderesin from deposited carbon resistors in order to enable the reuse ofthe cores.

A further object of the invention is to provide a method of removing adried paint consisting of silver and a phenol-formaldehyde resin fromdeposited carbon resistors which requires a minimum of labor and whichuses chemical ingredients which are economical, eflicient and readilyobtainable.

In accordance with the invention, the method includes immersing thearticle in a solution of nitric acid to dissolve the silver andimmersing the article in an aqueous solution of sodium hypochlorite toremove the resin from the article. Preferably, the first solutionconsists of concentrated nitric acid and the second solution consists ofdilute sodium hypochlorite and the article is washed after immersion inthe first solution to remove the nitric acid and the resulting silvernitrate.

This invention, together with furher objects and ad vantages thereof,will best be understood by reference to the following description of aspecific example thereof taken in connection with the accompanyingdrawing. The single figure in the drawing is a flow diagram for articlesfrom which dried paint consisting of silver and a phenolformaldehyderesin is to be removed in accordance with one embodiment of theinvention.

Example A large quantity of defective deposited carbon resistors coatedwith a baked paint consisting of about 86% metallic silver and about 14%of a phenol-formaldehyde resin binder was completely immersed, as at 1,in a solution of concentrated nitric acid. In the resulting reaction,the silver entered into solution as silver nitrate and was leached fromthe bake paint so as to leave the resin in a cellulated, irregularlyhoneycombed condition. After about two hours, the silver was dissolvedout of the baked paint. The resistors were allowed to remain immersedfor about 14 additional hours to further loosen and cellulate the resin.

The nitric acid was then removed and retained, and the resistors werewashed with water, as at 2, to remove the acid, silver nitrate and anyloose resin. Subsequently, the resistors were completely immersed, as at3, in a 5 to 10% by weight aqueous solution of sodium hypochlorite.After about 4 hours, most of the resin was removed. The resistors wereallowed to remain immersed for about 12 additional hours to effectcomplete removal of the resin. The solution of sodium hypochlorite wasthen re moved and retained. Subsequently, the resistors were washedthoroughly with water, as at 4, and dried, as at 5, in an oven at to C.

Ordinarily, sodium hypochlorite is considered a relatively non-reactiveagent at a dilute concentration and a low temperature, and would beexpected to have little elfect on a phenol-formaldehyde resin. However,contrary to expectations, a dilute solution of sodium hypochlorite atroom temperature does remove the resin effectively and efficiently.

It is believed that the honeycombed condition produced in the resin as aresult of the leaching out of the silver from the baked paint by thenitric acid accounts for the success of an unheated and dilute solutionof sodium hypochlorite. More specifically, it is believed that thehoneycombed condition allows the sodium hypochlorite to contact andreact with a much larger and thinner area of resin than if the resinwere a non-cellulate mass. Thus, relatively simple, readily obtainableand inexpensive chemicals can be used to remove the resin and withouthaving to be heated.

Examination of the treated resistors showed the baked silver andphenol-formaldehyde resin paint to have been substantially removed so asto present a clean carbon surface. The resistors were then suitable forrepainting or, if the carbon coating were defective, were ready forheating in an oxygen atmosphere to remove the carbon.

Neither the solution of nitric acid nor the solution of sodiumhypochlorite require heating. Although the speed of the reactions can beincreased by heating the reagents, such heating would requireunnecessary time and labor and would also tend to reduce the usable lifeof the reagents.

The time required to clean the resistors depends on the thickness andcomposition of the silver and phenolformaldehyde paint and the strengthof reagents. It has been found that in the case of typical resistorterminals the silver is removed by the first solution after about twohours, and the phenol-formaldehyde is removed by the second solutionafter about four hours.

Both the nitric acid and the sodium hypochlorite may be reused untilthey fail to clean properly. Their usable life is, obviously, a functionof the size and quantity of the articles to be cleaned, the quantity andcharacter of the silver and resin paint to be removed, and the quantityof nitric acid and of sodium hypochlorite used. It is, of course,required that the article, from which the dried paint is to be removed,be of such material that it is relatively inert to the two reagentsused.

While various embodiments and examples of the invention have beendisclosed, many modifications will be apparent, and it is intended thatthe invention be interpreted as including all modifications which fallwithin the true spirit and scope of the invention.

What is claimed is:

1. The method of removing a dried paint consisting of about 80 to 92% byweight metallic silver and about 20 to 8% by weight of aphenol-formaldehyde resin from deposited carbon resistors, whichcomprises:

immersing the resistors in an unheated solution of concentrated nitricacid for a period of at least 2 hours but not in excess of 16 hours toremove the silver from the resin such that the resin is a cellular mass;Washing the resistors to remove the nitric acid, and the resultingsilver nitrate and loosened resin; and

immersing the resistors in an unheated aqueous solution of 5 to 10% byweight sodium hypochlorite for at least 4 hours but not in excess of 16hours to remove the cellulated resin from the resistor.

2. The method of removing a dried paint consisting of about 80 to 92% byweight metallic silver and about 20 to 8% by weight of aphenol-formaldehyde resin from deposited carbon resistors, whichcomprises:

immersing the resistors in an unheated solution of con- 4 centratednitric acid for a period of 16 hours to remove the silver from the resinsuch that the resin is a cellular mass; Washing the resistors to removethe nitric acid, and the resulting silver nitrate and loosened resin;and immersing the resistors in an unheated aqueous solution of 5 to 10%by weight sodium hypochlorite for a period of 16 hours to removecellulated resin from the resistor.

References Cited in the file of this patent UNITED STATES PATENTS1,994,633 Boyd Mar. 19, 1935 2,208,294 Hempel July 16, 1940 2,572,576Stybel Oct. 23, 1951 2,637,670 Brown et al. May 5, 1953 3,047,508Fuentes July 31, 1962 FOREIGN PATENTS 834,707 Great Britain May 11, 1960OTHER REFERENCES Handbook of Plastics, Simonds and Ellis, 1943, D. VanNostrand C0., Inc. NY. (table on page 36 relied on).

1. THE METHOD OF REMOVING A DRIED PAINT CONSISTING OF ABOUT 80 TO 92% BY WEIGHT METALLIC SILVER AND ABOUT 20 TO 8% BY WEIGHT OF A PHENOL-FORMALDEYDE RESIN FROM DEPOSITED CARBON RESISTORS, WHICH COMPRISES: IMMERSING THE RESISTORS IN AN UNHEATED SOLUTION OF CONCENTRATED NITRIC ACID FOR A PERIOD OF AT LEAST 2 HOURS BUT NOT IN EXCESS OF 16 HOURS TO REMOVE THE SILVER FROM THE RESIN SUCH THAT THE RESIN IS A CELLULAR MASS; WASHING THE RESISTORS TO REMOVE THE NITRIC ACID, AND THE RESULTING SILVER NITRATE AND LOOSENED RESIN; AND IMMERSING THE RESISTORS IN AN UNHEATED AQUEOUS SOLUTION OF 5 TO 10% BY WEIGHT SODIUM HYPOCHLORITE FOR AT LEAST 4 HOURS BUT NOT IN EXCESS OF 16 HOURS TO REMOVE THE CELLULATED RESIN FROM THE RESISTOR. 